MONDAY, Nov. 21 (HealthDay News) -- Thin people appear to have a higher risk of dying within 30 days of an operation than heavier people, researchers have found.

The reason for the finding isn't clear, the study authors said. They set out to see what effect obesity had on survival after surgery, but instead found that the thinner a patient was, the greater the risk of death. The measurement the investigators used was body mass index (BMI), which takes into account both height and weight.

"Patients with low body mass index are at significantly higher risk of 30-day mortality following general and vascular surgical procedures," said lead researcher George Stukenborg, an associate professor in the department of public health sciences at the University of Virginia.

Body mass index is a significant predictor of death risk, independent of the differences in risk associated with the type of surgical procedure and other patient characteristics, he noted.

"Low body mass index should be recognized as an important risk factor for 30-day mortality and should be taken into account in preoperative decision-making, discharge planning and patient education," Stukenborg said.

The study was published in the Nov. 21 online edition of the Archives of Surgery.

For the study, Stukenborg's team looked at deaths in the month after operations in almost 190,000 people who underwent surgery in 2005 or 2006. The data came from the American College of Surgeons National Surgical Quality Improvement Program.

Of these patients, more than 3,200 died within 30 days after their operation. Of those with a BMI of less than 23.1, which put them in the normal-to-thin range, the percentage who died was 2.8 percent, compared with 1 percent among people whose BMI was 35.3 or higher (considered obese).

And those with a BMI of less than 23.1 had a 40 percent higher risk of dying than people with mid-range BMIs of 26.3 to 29.6 (overweight), the researchers found.

According to the U.S. Centers for Disease Control and Prevention, a BMI below 18.5 is underweight, between 18.5 and 24.9 is normal, 25 to 29.9 is overweight, and 30 and above is obese.

Patients who underwent exploratory abdominal surgery were at the greatest risk of death (nearly 14 percent), compared to other operations. The lowest risk was for breast lumpectomy, to remove small tumors (0.1 percent).

Other procedures where BMI appeared to be a factor included colostomy, wound cleaning, musculoskeletal system procedures, upper gastrointestinal procedures, colorectal surgery and hernia repair, the study authors noted.

"Unfortunately, our research does not shed any light on why BMI is a risk factor for mortality," Stukenborg said. "This is an interesting question, though, and something we should think more about," he added.

Commenting on the study, Dr. Nestor de la Cruz-Munoz, chief of bariatric surgery at the University of Miami Miller School of Medicine, said that he was not surprised by the finding. "It's kind of something we suspected but no one has ever looked at it in a big series like this," he said.

It makes sense that underweight people have a greater risk of dying, de la Cruz-Munoz noted.

"A lot of these patients are malnourished -- maybe cancer patients, patients undergoing treatment for other medical problems. A lot of time these patients don't have the defenses to do well with a major surgery," he said. "These patients are not your skinny young girl, but more like a frail 80-year-old woman."

SOURCES: George J. Stukenborg, Ph.D., associate professor, department of public health sciences, University of Virginia School of Medicine, Charlottesville, Va.; Nestor de la Cruz-Munoz, M.D., chief of bariatric surgery at University of Miami Miller School of Medicine; Nov. 21, 2011, Archives of Surgery, online

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